The invention is based on a mains filter having two coils arranged on a common core.
Filters of this kind are used, in particular, at the input in switched-mode power supplies for connection to the mains supply. They not only prevent high-frequency interference produced in the switched-mode power supply from reaching the mains supply via the mains connection but also prevent high-frequency interference from reaching the appliance via the mains supply, whilst allowing the 50-Hz mains frequency through essentially unattenuated.
In this arrangement, the coils are usually produced on a cylindrical coil former which is divided into chambers and has windings made from a copper wire, a core being passed through the coil former. The operating principle of such a filter is explained with reference to FIG. 1. In this case, each wire in the mains line is passed through a respective coil C1 or C2, the two windings C1, C2 having opposite polarities, with reference to the two inputs E1 and E2. This means that high-frequency interference propagating on one input line, for example over input line E1, is subjected to the full inductance of the coil C1. On the other hand, however, at the mains supply""s 50-Hz frequency, which is applied to the two inputs E1, E2 simultaneously, the different polarities of the two windings cause the magnetic fields in the core to be cancelled out. The filter should therefore be of symmetrical design, so that no input impedance is formed for the 50-Hz frequency. The two coils C1, C2 are coupled to one another magnetically by means of a common core (not shown).
The object of the present invention is to specify a filter of the type mentioned above which has good electrical properties together with compact dimensions.
The mains filter of the invention comprises coils with windings which are arranged on substrates, for example conventional thin printed circuit boards, as conductor tracks. In this arrangement, a coil can contain one or more substrates, which are in contact with one another in the case of a plurality of substrates. In the centre, the substrates have an opening through which a core is passed.
A substrate comprises, in particular, a plurality of windings which are routed spirally inwards and through a plated-through hole to the other side of the substrate. On the reverse, the winding is routed spirally outwards again, so that the two connections for the winding can be made at the edge of the circuit board. In particular, this means that the input and the output for a coil can also be arranged on opposite sides of the circuit board. With an appropriate number of windings, one substrate is sufficient for one winding, so that only one substrate per coil is required for the filter. The two substrates for the two coils can, in particular, be of identical design, the input and the output for a coil being arranged on a substrate in the region of opposite corners. Symmetrical considerations mean that an E/E core or an E/I core is advantageous.
With plated-through holes, insulation between the two coils is necessary. For this, a dielectric interlayer, for example a plastic film, can be used. However, it is also possible to coat the opposite sides of the substrates accordingly with a dielectric material. The insulation requirements for a mains filter are significantly lower than for a transformer. This means that virtually the full width of the substrates can be used in the region inside the openings in the core.
Both the core and the substrates with the conductor tracks arranged on them can be manufactured with very high precision. In this respect, the substrates can be dimensioned such that they are held without play by the core alone, particularly its openings. A coil former is therefore not required. Known etching methods are used to arrange the conductor tracks, on the substrates, in particular extremely symmetrically, so that a mains filter with substrates has significantly better electrical properties than one with a coil former having a copper wire winding with a great deal of asymmetry.
The core used can be a narrow E/E core or E/I core, in particular, so that the filter is very compact and can be arranged perpendicularly on a circuit board in a power supply unit using appropriate retaining means, the space requirement on the circuit board being very low. In many appliances, the switched-mode power supply is arranged in a screened metal cage as an external unit with a circuit board. Since a switched-mode power supply frequently uses two mains filters, this means that the size of the power supply unit is significantly reduced.
The filter is used, in particular, for current-compensated mains filters in switched-mode power supplies, for example in consumer electronic appliances.